The effect of oiling the insulation of power transformers on the efficiency of the final vacuum cycle

This paper reports and discusses results from small-scale laboratory tests investigating the moisture transients of different cellulosic insulation components subjected to selected conditions and drying procedures. The effect of water adsorption during the final clamping of the transformer active part, prior to the ultimate drying cycle, was simulated. The merit of oiling the insulation, a measure that slows down the water adsorption rate and the water diffusion significantly, was examined systematically. The macroscopic water gradients from the surface to the interior of pressboard and crepe paper insulation were determined, using the Karl Fischer titration method. It has been found that the water ingress to previously dried paper and pressboard insulation is strongly dependent on time, type of insulation and pressboard ply orientation relative to the air-exposed surface. The effectiveness of oiling the cellulosic transformer insulation is depending on the exposure time elapsing during the final assembly work, and varies with the type of insulation. The impact of the relevant parameters on the water extraction behavior during the subsequent, fine vacuum cycle is reported in detail